(1) Field of the Invention
The present invention relates to a video output apparatus which sends a video signal to a receiving apparatus such as a television receiver, and particularly relates to a video output apparatus in which the color format of the video signal can be switched.
(2) Description of the Related Art
In recent years, devices that handle digital video signals have become common due to the spread of Digital Versatile Disks (DVDs) and digital television broadcasting.
The High-Definition Multimedia Interface (HDMI) standard can be given as an example of an interface which connects a sending device that sends a video signal (i.e. a DVD player) with a receiving device that receives the video signal (i.e. a television). HDMI is an interface for transmitting baseband digital video signals to receiving devices, and is being implemented in consumer devices while maintaining backwards compatibility with the Digital Visual Interface (DVI) standard widely used in personal computers (PCs). A single HDMI cable connects the sending device with the receiving device, and transmits baseband video and audio data. The supported color format of this transmittable video signal is not limited only to the RGB signals used in PCs; HDMI also supports luminance and chroma signals (hereafter referred to as “YCbCr” signals), which are generally used in television receiving devices.
FIG. 1 is a block diagram showing a simplified configuration of a system connected via HDMI. This system includes a sending device 10 and a receiving device 20. The sending device 10 and the receiving device 20 are connected via an HDMI cable. An HDMI sending unit 700 included in the sending device 10 sends video data and audio data to the receiving device 20 via the HDMI cable. Identification information packets are multiplexed with the video signal. The identification information packets indicate whether the video data is an RGB signal or a YCbCr signal.
FIG. 2 is a diagram showing the position, within the video signal, of the identification information packets multiplexed with the video signal. In FIG. 2, a progressive image with a resolution of 720×480 pixels is shown as an example. Furthermore, a single identification information packet is multiplexed in each frame during the vertical blanking interval (VBI) of the video signal.
The sending device 10 can, via remote-control operations or the like performed by a user, switch the color format of the video signal being sent to a desired color format, in accordance with the abilities of the receiving device 20 or based on the user's preferences.
FIG. 3 is a block diagram showing a configuration of the HDMI sending unit 700, which is capable of switching the color format of the video signal. As shown in FIG. 3, the HDMI sending unit 700 includes a video input unit 701, a color conversion unit 702, an identification information packet issuing unit 703, and a video output unit 704, and switches the color format via a CPU 710 provided within the sending unit 10.
The video input unit 701 inputs YCbCr or RGB video data.
When conversion has been instructed by the CPU 710, the color conversion unit 702 converts the YCbCr signal to an RGB signal in the case where a YCbCr signal has been obtained from the video input unit 701, or converts an RGB signal to a YCbCr signal in the case where an RGB signal has been obtained from the video input unit 701. However, in the case where conversion has not been instructed by the CPU 710, the video signal obtained from the video input unit 710 is not converted, and is outputted as-is to the video output unit 704.
The identification information packet issuing unit 703 generates identification information packets which indicate whether the video signal is an RGB signal or a YCbCr signal. The receiving device 20 performs color conversion in accordance with the identification information packets in order to display the image.
The video output unit 704 multiplexes the video signal obtained from the color conversion unit 702 with the identification information packets, and transmits the multiplexed signal over the HDMI cable in accordance with the HDMI specification.
FIG. 4 is a diagram showing a control sequence for switching the color format performed in the HDMI sending unit 700. FIG. 4 shows a control sequence executed in the case of switching from a YCbCr signal to an RGB signal. The horizontal axis indicates time. In addition, the top four signals are signals on the sending device 10 side, while the bottom two signals are signals on the receiving device 20 side. In other words, FIG. 4 shows the following items, in order from the top down: commands outputted by the CPU 710 to the HDMI sending unit 700; inputted video data inputted to the color conversion unit 702; video data outputted by the color conversion unit 702; identification information packets issued by the identification information packet issuing unit 703; received video data received by the receiving device 20; and received identification information packets.
Upon detecting a remote control operation for switching the color format, the CPU 710 outputs, to the color conversion unit 702, a switch command indicating that the YCbCr signal is to be switched to an RGB signal (T1), and then outputs, to the identification information packet issuing unit 703, a switch command indicating that the identification information packets indicating a YCbCr signal are to be switched to identification information packets indicating an RGB signal (T2). The difference in timing of the two switch commands spans several frames. The reasons for this difference in timing between the two switch commands are as follows. First, the CPU 710 operates not only the HDMI sending unit 700 but also the sending unit 10 as a whole, and thus the period of the switch command increases in accordance with the processing load required for controlling the entire sending unit 10. Furthermore, the processing power of the CPU 710 is lower than that of PCs or the like.
Due to the timing difference between the two switch commands, a timing difference also arises between the timing at which the video data outputted from the color conversion unit 702 is switched and the timing at which the identification information packets issued from the identification information packet issuing unit 703 are switched. As a result, the timing difference travels through the HDMI cable, and a difference between the timing at which the received video data is switched and the timing at which the received identification information packets are switched (the period “Ta” in FIG. 4) arises in the receiving device 20.
During the period Ta, in which the color format of the received video data and the color format indicated by the received identification information packets differ, the receiving device 20 mistakenly interprets the received video data as being of the color format indicated by the received identification information packets, and displays the video data. The result is color scrambling during the period Ta.
In order to mask visual noise arising when the video format of a television broadcast is switched from progressive to interlaced and vice versa, Japanese Laid-Open Patent Application No. 2000-333088 discloses a technique in which the video is muted for a period starting from before the video format switch and continuing until after the video format switch.
However, there is a problem with the above conventional art in that while it is possible to mask visual noise arising when switching between video formats with different scanning methods, it is not possible to mask the color scrambling that arises when switching between video formats that have different color formats. In the example given in FIG. 4, color scrambling spanning several frames occurs in the video display when switching the color format, and while the scrambled colors occurs only for an instant, it is sufficient to give a user a sense of discomfort and cause the user to worry that the sending device or the receiving device may have malfunctioned.
FIG. 5 is a table contrasting the color data of RGB and YCbCr signals in regards to black and white colors. In FIG. 5, black and white color values are shown for a 24-bit RGB signal and a 24-bit YCbCr signal respectively. In the RGB signal, black is expressed by the values (R, G, B)=(0, 0, 0), while the white is expressed by the values (255, 255, 255). On the other hand, in the YCbCr signal, black is expressed by the values (Y, Cb, Cr)=(16, 128, 128), while white is expressed by the values (235, 128, 128). For example, if an RGB signal expressing black is mistakenly interpreted as a YCbCr signal, or vice versa, an image of differing color and brightness will be displayed. When colors are interpreted incorrectly between color formats which express colors differently, scrambled colors will occur.
FIG. 6 is a table showing combinations in which scrambled colors occur. Each row in FIG. 6 indicates the pre-switch color format, whereas each column indicates the post-switch color format. “4:4:4” for the YCbCr signal indicates that the ratio between the number of pixels for the luminance signal Y, the number of pixels for the chroma signal Cb, and the number of pixels for the chroma signal Cr is 4:4:4. In the same manner, “4:2:2” indicates that this ratio is 4:2:2. “8-bit” for the RGB signal indicates that each RGB component is of 8 bits. In the same manner, “12-bit” indicates that each RGB component is of 12 bits. In FIG. 6, scrambled colors do not occur in color format conversion between RGB signals for which the bit numbers for components differ and in color format conversion between YCbCr signals for which the pixel number ratios differ. On the other hand, there is a problem in that scrambled colors when switching color formats between RGB and YCbCr signals.
It can be thought that scrambled colors will not occur if, for example, there is no difference in the timing between the command issued from the CPU instructing the color format switch and the command issued from the CPU instructing the identification information packet switch. However, even if there is no difference in the timing, the problem of scrambled colors occurring still exists in cases where the receiving device has low processing power, the receiving device cannot synchronize its interpretation of the identification information packets with a change in the color format of the video signal, and other such cases.
Accordingly, an object of the present invention, which has been conceived in light of the abovementioned problems, is to provide a video signal sending device which reduces the occurrence of scrambled colors when switching video signals between different types of color formats, thereby alleviating a sense of discomfort or worries of malfunctions on the part of the user.